Bump NumPy to 1.15

[WillAyd]

Follow up to #29212 we should bump Numpy to 1.15

Optional: we hit CI failures in the PR above on Python 3.6.0 related to Numpy. We bumped Python to 3.6.1 but if we bump numpy instead 3.6.0 might work, if we even care

[gfyoung]

Same thoughts as in #29512 (comment)

[WillAyd]

Closing in favor of #29512